Interestingly, the lattice parameters also depend linearly on the stoichiometry in hypo — and hyper­stoichiometric actinide (mixed or not) dioxides. The variation of the volume AO/O induced by single
isolated defects may be related to the variation of the macroscopic volume A V/ V, as follows:

AV AO

/ [8l

V O M

Depending on the sign ofAO, there will be a swelling or a contraction of the volume. The evolution of the cell parameter is linear as function of stoichiometry x in UO2 + x and (U, Pu)O2 ± x (see Figure 14), with different slopes for hypo — and hyperstoichiometric dioxides from Javed,86 Gronvold26 in UO2 + x, and Markin et a/.53 in (U, Pu)O2 ± x. This is because the formation volumes AO of oxygen defects are different. Concerning the thermal expansion, the coefficients are roughly similar to each other, whatever be the stoichi­ometry considered, as can be seen in Figure 15. Such a behavior is well reproduced by MD calculations (see Watanabe eta/.87 and Yamasaki eta/.88).

The evolution of the lattice parameters as a func­tion of stoichiometry has been summarized by Kato et a/.56 in minor actinides containing MOX (see Figure 16(b)). The hypostoichiometry induces a swelling of the lattice as in UO2 + x. Such behavior has been seen in americium containing PuO2 by

Temperature (K)

Miwa eta/.90 and in pure MOX (see Arima eta/.,81 and references therein), and this is qualitatively repro­duced by MD calculations (see Figure 16(a)). The thermal expansion coefficients of hypostoichiometric dioxides (not reproduced here) simply follow the Vegard’s law (see Arima et a/.81), as evidenced in urania in the previous section.